The present invention relates to a ferroelectric film, a method of manufacturing a ferroelectric film, a ferroelectric capacitor, and a ferroelectric memory.
In recent years, a ferroelectric film such as Pb(Zr,Ti)O3 (PZT) or SrBi2Ta2O9 (SBT), a ferroelectric capacitor using the ferroelectric film, and a ferroelectric memory device have been extensively researched and developed. The structure of the ferroelectric memory device is roughly divided into a 1T type, 1T1C type, 2T2C type, and simple matrix type. A 1T type ferroelectric memory device has such a structure that the retention time (data retention time) is as short as one month due to occurrence of an internal electric field in the capacitor. Therefore, it is considered that the 1T type ferroelectric memory device cannot ensure a 10-year guarantee generally required for semiconductor devices. A 1T1C type or 2T2C type ferroelectric memory device has a configuration approximately the same as that of a DRAM, and includes a select transistor. Therefore, the DRAM manufacturing technology can be utilized, and a write speed approximately equal to that of an SRAM can be realized. Therefore, small capacity products having a capacity of 256 kbits or less have been produced on a commercial basis.
PZT has been mainly used as the ferroelectric material for the 1T1C type or 2T2C type ferroelectric memory device. As the ferroelectric material, PZT which has a composition in or near the mixed region of the rhombohedral crystal and the tetragonal crystal, in which the Zr/Ti ratio is 52/48 or 40/60, and is doped with an element such as La, Sr, or Ca, has been used. The above region is used in order to ensure reliability which is most required for a memory device.
Specifically, an excellent hysteresis shape is obtained in the Ti-rich tetragonal region. However, a Schottky defect occurs in the tetragonal region due to the ionic crystal structure. This causes leakage current characteristics or imprint characteristics (degree of change in hysteresis shape) to deteriorate, so that it is difficult to ensure reliability. Therefore, PZT having a composition in or near the mixed region of the rhombohedral crystal and the tetragonal crystal is used as described above.
A simple matrix type ferroelectric memory device has a cell size smaller than that of the 1T1C type or 2T2C type ferroelectric memory device, and enables multilayering of capacitors. Therefore, the simple matrix type ferroelectric memory device is expected to enable an increase in the degree of integration and a reduction in cost. A related-art simple matrix type ferroelectric memory device is disclosed in JP-A-9-116107, for example. JP-A-9-116107 discloses a drive method in which, when writing data into a memory cell, a voltage one-third of a write voltage is applied to unselected memory cells.
A hysteresis loop having excellent squareness is indispensable in order to obtain a simple matrix type ferroelectric memory device. As a ferroelectric material which can deal with such a requirement, Ti-rich tetragonal PZT can be given. However, it is difficult to ensure reliability by using Ti-rich tetragonal PZT in the same manner as the 1T1C type or 2T2C type ferroelectric memory.